As is known in the art, a radar system generally includes an antenna, a transmitter and a receiver. In general overview, the transmitter generates an electromagnetic signal which is emitted or radiated through the antenna. The radiated electromagnetic signal propagates in a predetermined region of space and intercepts one or more objects in the path of the electromagnetic radiation. Portions of the electromagnetic radiation reflect off the objects and propagate back towards the radar system where the reflected signals are detected by the receiver. Such reflected signals are sometimes referred to as return or echo signals.
If the radar system employs a directive antenna, a relatively narrow beam of electromagnetic radiation is emitted and the direction from which the return signals propagate and hence the bearing of the object may be estimated. The distance or range to the reflecting object can be estimated by transmitting signal pulses and measuring the time period between the transmission of the transmitted pulse and reception of the return signal pulse.
One particular type of radar system is a monopulse radar system. A monopulse radar system refers to a radar system which obtains a complete measurement of an object's angular position by transmitting a single signal pulse and receiving the corresponding return or echo pulse. Together with a range measurement performed with the same pulse, the object position in three dimensions is determined completely. Typically, a series or train of echo pulses is employed to make a large number of repeated measurements and produce a refined estimate of the object's position.
A monopulse receiving system typically includes a monopulse circuit which receives signals from the antenna and forms sum and difference monopulse output signals. The sum and difference signals are formed by combining received antenna signals in a particular manner. The signals can be combined using circuits referred to as hybrid circuits. The hybrid circuits may be provided as so-called magic-T or rat race circuits which receive signals fed thereto and add and/or subtract the signals in a known manner. Such hybrid circuits can be fabricated using either printed circuit or waveguide transmission lines.
To determine the location of an object in a single angular coordinate (e.g. either azimuth or elevation), the monopulse circuit need only include a single hybrid circuit and thus the monopulse circuit is relatively compact. To determine the location of an object in two angular coordinates (e.g. both azimuth and elevation), the monopulse circuit requires multiple hybrid circuits. Thus, conventional monopulse circuits capable of determining the location of an object in two angular coordinates can become relatively large.
The monopulse sum and difference signals can be formed either at the transmitted signal frequency or, after down conversion of a return signal, at a lower frequency. The transmit signal frequency is typically in the microwave or millimeter wave frequency range. When the monopulse sum and difference signals are formed at the transmitted signal frequency, the monopulse is typically coupled directly to the antenna with relatively few, if any, circuits disposed between the antenna output ports and the monopulse input ports. The operations to generate monopulse sum and difference signals typically are performed at microwave or millimeter wave frequencies by the hybrid circuits which are typically fabricated using either printed circuit or waveguide transmission lines.
Obtaining the sum and difference signals at the transmitted signal frequency (i.e. before any down conversion) reduces the amount of additional errors which may be otherwise introduced into the signals used to form the monopulse signals by circuits (e.g. mixer circuits) coupled between the antenna output ports and the monopulse input ports. For example, to form the monopulse signals after down conversion of a return signal to a lower frequency it is necessary to couple a mixer or other frequency translation device between the antenna output ports and the monopulse input ports. Practical frequency translation devices (e.g. mixer circuits) introduce errors into the signals which are combined in the monopulse circuit to provide the monopulse output signals.
Typically, a single sum channel and a pair of difference channels are formed by the monopulse circuit to allow resolution of two angular coordinates. In systems which utilize a conventional waveguide multimode horn feed, a waveguide monopulse network can process a radar return signal to generate monopulse sum and difference signals which propagate in appropriate monopulse sum and difference channels. The radio frequency (RF) signals propagating through the monopulse channels are converted to intermediate frequency (IF) signals using waveguide mixers. The IF signals are fed to an IF receiver for additional processing.
One problem with this RF waveguide approach to implementing the monopulse network is that the monopulse circuit is relatively large and must be fabricated using relatively expensive and time consuming precision machining or electroforming techniques. This is particularly true in those system which operate in the millimeterwave frequency range. To overcome this drawback, systems operating at millimeter wavelength frequencies can downconvert received signals to an intermediate frequency prior to monopulse processing. With this approach, monopulse processing may be performed at the intermediate frequency in lieu of monopulse processing performed at the higher fundamental or transmit frequency. While the circuit fabrication tolerances are generally less severe at lower frequencies, there is a concomitant increase in the size of waveguide circuit components. Thus, the use of waveguide transmission lines to process and convert the monopulse information (especially at millimeter wave frequencies) is not a practical low cost solution suitable for high volume production.
To further complicate matters, projectiles such as missiles and submunitions having a relatively small diameter require relatively high resolution monopulse receivers to enable accurate tracking of a target. Conventional monopulse receiving systems operating in the 1 gigahertz (GHz) to 20 GHz frequency range do not provide the angular resolution needed to accurately track targets. Furthermore, the size of RF circuit components which operate in the 1 GHz to 20 GHz range are physically too large and cumbersome to be packaged in many small projectiles. Therefore, operation at millimeter wave frequencies above 30 GHz is required.
Missile seeker systems having a relatively large diameter typically operate at microwave frequencies and form monopulse output receive signals with comparator networks provided from hybrid circuit components implemented using stripline, coaxial or waveguide transmission media. The monopulse output signals are typically fed to amplifiers having a relatively high gain and a relatively low noise figure. The amplified signals are subsequently downconverted to an appropriate intermediate frequency (IF) by a radio frequency (RF) microwave mixer module. For those applications in which the monopulse receiver must be disposed in a projectile having a relatively small diameter, however, the signal transmission losses and overall size of conventional receiver systems adversely impact seeker performance. Operation at higher frequencies such as millimeter wave (MMW) frequencies is a necessity to achieve the requisite resolution but there are limitations in the availability of receiver devices which operate at such frequency bands. For example, it is relatively difficult and expensive to provide RF devices having the performance characteristics (e.g., noise figure, power handling, power limiting, etc.) required for efficient active seeker operation in the MMW frequency range.
The complexity of radar systems operating in the millimeter wave frequency band will be appreciated when it is recognized that at an operating frequency of 94 GHz, for example, dimensions of a conventional rectangular waveguide are in the order of 0.050 to 0.100 inches, with tolerances of better than 0.001 inches required in many critical assemblies. Although it may be possible to fabricate such millimeter-wave waveguide structures at somewhat reduced cost using modem fabrication techniques, the expense associated with tuning and testing such critically toleranced hardware is often cost prohibitive.
Furthermore, the problems of packaging and tuning a millimeter-wave seeker in a conventional submunition will be appreciated when it is recognized that a monopulse seeker with a monopulse tracking capability utilizing waveguide components may well require in excess of twenty different waveguide components to control the routing and duplexing of the various signals coming from the transmitter and returning to the receivers. If a monopulse capability were required, then all of the foregoing waveguide components would be required to track from channel to channel in both amplitude and phase.
At an operating frequency of 94 GHz, each one thousandth of an inch in the length of a waveguide transmission line is equivalent to about 2.degree. of phase. It should, therefore, be appreciated that it is relatively difficult to obtain inexpensively the requisite phase and amplitude tracking between various receiver channels.
Another problem inherent in millimeter-wave radar seekers utilizing waveguide devices is that of providing sufficient isolation between a transmitter and receiver. This problem is exacerbated by the fact that waveguide switches and circulators which can withstand relatively high power transmit signals and provide a high degree of isolation are not generally available in a compatible size at relatively high operating frequencies.
It would, therefore, be desirable to provide a relatively compact monopulse receiver having a relatively low noise figure which operates in the millimeter wave frequency range and which can operate in a system which includes a transmitter which transmits signals having relatively high power levels.